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Introduction

Transport between stratosphere and troposphere across tropopause can take place by many dynamical processes. These include global scale Brewer-Dobson cell, quasi-isentropic mixing by large scale eddies, tropical cumulonimbus cloud penetration in to the stratosphere, tropopause folds associated with mid-latitude baroclinic instability, wave activity etc. Mainly tropospheric mass enters into the stratosphere in the equatorial region by the rising branch of the Brewer-Dobson cell, cumulus cloud penetration etc. On the other hand, stratospheric air enters the troposphere by the tropopause fold phenomena and other synoptic and smaller scale circulation processes. Recently more attention has been shifted from synoptic and smaller scale stratosphere ­ troposphere exchange to global scale in which stratospheric zonal mean circulation plays a major role (Holton et al, 1995). Number of observational evidences suggests that the lower part of the extratropical circulation cannot be simply explained by this global scale mean circulation (Wirth and Egger, 1999). Complete understanding of the smaller scale stratosphere ­ troposphere exchange processes are important for the chemical and ozone distribution in the upper troposphere and lower stratosphere.